1. Field of the Invention
The present invention relates to compressive coding devices which in particular perform irreversible compressive coding on visual data subjected to transparency control. The present invention also relates to visual display control devices which decode compressive coded data so as to display graphics on screen.
The present application claims priority on Japanese Patent Application No. 2009-156990 (filing date: Jul. 1, 2009), the content of which is incorporated herein by reference.
2. Description of the Related Art
Sprite-mode visual processing LSI (Large Scale Integration) devices are typical examples of visual processing devices used for game devices. This type of visual processing devices performs a series of procedures in which a sprite pattern memory stores visual data of sprites (i.e. independent graphic objects) representing game characters; visual data are read from the sprite pattern memory in conformity with the progression of a game; visual data are edited via rotation and scaling (i.e. expansion and reduction) and written into a buffer; thereafter, images and graphics are displayed on the screen of a liquid crystal display (LCD) based on the stored contents of the buffer. This type of visual processing devices performs an overlay representation in which the background image is overlaid with sprites such as game characters. The overlay representation is implemented by way of transparency control in which pixels corresponding to the background of each sprite are rendered in transparency and visualized on the screen. In the transparency control, visual processing devices choose colors of pixels each rendered in transparency in advance so as not to display pixels whose data indicate the transparency on the screen.
Normally, sprite-mode visual processing devices perform compressive coding on visual data stored in sprite pattern memory. The compressive coding technology is generally classified into reversible compressive coding and irreversible compressive coding. The reversible compressive coding is a combination of predictive coding and variable-length coding, for example. The reversible compressive coding is able to perfectly restore original visual data (i.e. visual data prior to compressive coding) based on compressive coded data (i.e. visual data already subjected to compressive coding) but unable to obtain a high compression factor. The irreversible compressive coding employs orthogonal transformation, for example. The irreversible compressive coding is able to obtain a high compression factor but unable to perfectly restore original visual data based on compressive coded data. For this reason, the reversible compressive coding and the irreversible compressive coding are alternately changed according to needs. That is, the reversible compressive coding is employed when it needs to perfectly restore original visual data, while the irreversible compressive coding is employed when it needs to reduce the amount of compressive coded data.
In the case of compressive coding on visual data subjected to transparency control, it is preferable to alternately change the reversible compressive coding and the irreversible compressive coding upon considering the necessity of reducing the amount of compressive coded data. The irreversible compressive coding on visual data subjected to transparency control suffers from drawbacks in which pixels ascribed to transparency control are not rendered in transparency, and pixels not ascribed to transparency control are rendered in transparency. For example, FIG. 14A shows that the transparency is designated by “0xFFFFFF” (where primary-color components R, G, and B are each set to “0xFF”), wherein pixel data ascribed to transparency control is changed into “0xFEFEFE” due to the irreversible compressive coding and decoding so that it is not rendered in transparency. FIG. 14B shows that the transparency is designated by “0x000000”, wherein input pixel data of “0x0101010” not ascribed to transparency control is changed into “0x000000” due to the irreversible compressive coding and decoding so that it is rendered in transparency. These drawbacks are originated in the property of the irreversible compressive coding in which original visual data cannot be perfectly restored based on compressive coded data.
Patent Documents 1 and 2 disclose solutions to the above drawbacks, wherein compressive coded data are attached with mask data, in which binary data discriminating whether pixel data are subjected to transparency control or not are aligned along with the sequence of pixels. As shown in FIGS. 14C and 14D, an additional bit is added to each pixel data already subjected to irreversible compressive coding so as to discriminate whether each pixel data is subjected to transparency control or not. The additional bit of “1” indicates that each pixel data is subjected to transparency control while “0” indicates that each pixel data is not subjected to transparency control. Using the additional bit, it is possible to easily discriminate pixel data whether they are subjected to transparency control or not. Using the additional information such as mask data and additional bits, it is possible to precisely perform the transparency control in a decoding procedure after the irreversible compressive coding on visual data representing transparent pixels.                Patent Document 1: Japanese Patent Application Publication No. H11-205788        Patent Document 2: Japanese Patent Application Publication No. 2003-87572        
In the above technology in which the additional information is attached to compressive coded data, it is necessary to evaluate the overall compression factor considering the amount of the additional information. Compared with the conventional technology in which the additional information is not attached to compressive coded data, this technology suffers from a reduction of the compression factor. Since the irreversible compressive coding has an advantage yielding a high compression factor compared with the reversible compressive coding, the additional information may impair the advantage of the irreversible compressive coding.